A motor vehicle generally utilizes a propeller shaft (also referred to as a driveshaft or propshaft, and for brevity the term propshaft will be used hereinafter) to transfer the rotating mechanical force (torque) generated by the engine/transmission to the driving wheels of the vehicle, which, in turn, propel the vehicle. For example, a propshaft is used for connecting the transmission or transfer case (in the case of four or all wheel drive vehicles) to the driving axle(s). Propshafts can either be single piece, as for example shown at FIGS. 1A and 1B, or multi-piece, as for example shown at FIG. 1C.
A single piece propshaft assembly 10 includes a shaft 12, which is usually a tube of metal material M having a length L and a radius R; a pair of articulating joints 14, 16 at each end 12a, 12b of the shaft; and an axial slip interface 18, usually via splines, at, usually, only one or the other of the articulating joints. In this manner, the ends of the shaft drivingly connect components of the motor vehicle, as for example a transmission 20 to a rear drive axle 22. Alternatively, the prior art has also used splines at the ends of the propshafts as the means of connecting to the end yokes, whereby the need to utilize welds to join these components is eliminated.
A multi-piece propshaft assembly 30 has first and second shafts 32, 34, which are connected through a bearing support 36, wherein distal ends 32a, 34b of the first and second shafts are connected, respectively, to an articulating joint 38, 40. The bearing support 36 is affixed to a frame member of the motor vehicle and bearingly supports a splined stub shaft 44 which is attached to one end 32b of the first shaft 32 and interfaces with a splined slip yoke 45 which supports a third universal joint 46 that is connected with the second shaft 34. The splined interface forms a slip mechanism 48. In this manner, the ends of the first and second shafts drivingly connect components of the motor vehicle, as for example a transmission 50 to a rear drive axle 52.
A relatively rare propshaft design is the torque tube assembly 60, for example shown at FIG. 1D. This design incorporates a rotating shaft 62 that may be a solid rod or a hollow tube that transmits power and is supported by multiple bearings 64 encased by a stationary, rigid outer tube or beam 66 that is rigidly anchored 68a, 68b at each end to a respective drive component 70, 72. The torque tube assembly 60 has been used in automotive applications over the years where there is no relative motion between the ends, as for example in one form or another in the General Motors Corvette and the Porsche 944 and 928. In recent years, the outer tube has acted as a powerplant structural component as seen on sports car vehicle applications where the torque tube connects the engine to the transmission. In American vehicles of the 1930s and 1940s, a torque tube assembly was rigidly anchored to the driving axle and to the transmission with the addition of a “torque ball joint” that allowed angular misalignment between the torque tube shaft and the transmission shaft to occur near the connection at the transmission. This arrangement does not permit operation between any non-parallel shafts as it requires the shafts to intercept at the joint. Rigidly anchoring the torque tube to a solid beam driving axle and powerplant has significant disadvantages because the mass and inertia of the powerplant have influence on vehicle handling dynamics.
With regard to considerations of propshafts and their relation to the motor vehicle architecture, the space in the underbody of a vehicle is at a premium in order to maximize space for passengers and cargo, so it is desirable from that standpoint for the diameter of propshaft to be minimized while still meeting mechanical requirements of the propshaft. It is also desirable to minimize the weight of vehicle components, including the propshaft, in order to improve performance and fuel economy.
Of concern in the selection of propshafts is that they can become dynamically unstable if operated at rotational speeds where the propshaft residual imbalance forces excite the propshaft bending modes of vibration, also known as the propshaft natural bending frequencies. The rotational speed coinciding with the first bending mode of vibration (natural bending frequency) is known as the propshaft critical speed. The low dampening properties of typical materials used in the construction of propshafts result in bending stresses that quickly increase at resonance and can cause a propshaft fracture.
Factors influencing the resonance bending frequency are selection of material, the diameter, and the length of the propshaft, wherein as the length increases, the resonance frequency decreases, and so does the critical speed. Therefore, longer propshafts typically require the use of larger diameter, lighter and more rigid materials to meet critical speed requirements.
FIG. 1E is a graphic representation 80 of the appearance of propshaft bending stress (pounds per square inch) as a function of propshaft rotation speed, indicative of the propshaft critical speed A, B, C for various designs of propshafts. In this regard, propshaft critical speed A is indicative of a large diameter propshaft, and the other critical speeds B and C are for progressively small diameter propshafts.
A conventional design propshaft is not allowed to operate at or very near its critical speed. In this regard, the operational rotation speed range A′ is for propshaft critical speed A; the operational rotation speed range B′ is for propshaft critical speed B; and the operational rotation speed range C′ is for propshaft critical speed C.
Multi-piece propshafts have been used to overcome the problems associated with a long single piece propshaft. Each shorter piece has a comparatively higher resonance frequency. Overall, this gives the multi-piece propshaft a higher critical speed.
While multi-piece propshafts overcome some of the deficiencies of single piece propshafts, they introduce other problems. For example, multi-piece propshafts tend to exhibit launch shudder. Launch shudder is a severe vibration that occurs when the vehicle starts from a standstill or is driven at low speed during high powertrain torque events. In the case of a multi-piece (two piece) propshaft as discussed above with respect to FIG. 1C, a stationary center bearing support and an additional joint are introduced where the two pieces of the propshaft come together. The center joint allows the rear propshaft piece to operate at different angles from the front piece. Typically, the stationary center support attaches the front piece of the propshaft to the vehicle chassis or body structure. Since the transmission is also attached to the vehicle structure, the front piece of the propshaft does not move much during vehicle operation. However, the same cannot be said for the rear piece of the propshaft on vehicles with solid beam drive axles with Hotchkiss (leaf spring) or link coil spring suspension systems. Relative to the vehicle, the position of the rear propshaft joint moves vertically (see V in FIG. 1C) during operation of the vehicle. For example, when a vehicle's payload is increased or decreased, the rear joint moves as the vehicle body moves toward or away from the axle. This movement alters the joint operating angles (see α in FIG. 1C) on the propshaft piece attached to the axle.
Launching a typical vehicle with a multi-piece propshaft with universal joints and a Hotchkiss suspension, the leaf springs deflect under the influence of the driving torque changing the orientation of the axle and increasing joint operating angles. Applying driving torque to a universal joint operating at angle generates secondary couples perpendicular in direction to the driving torque. The magnitude of these couples increase with increased angle and increased driving torque. Furthermore, the couple magnitudes oscillate at a frequency of twice per propshaft revolution and exert oscillating forces at every propshaft support. These oscillating forces are the cause of the launch shudder vibration. Launch shudder severity typically increases with higher payload, heavier throttle application, trailering and operating the vehicle up a grade.
Launch shudder is much less likely to occur in single piece propshafts because the degree of freedom introduced by the center joint is eliminated, the joint operating angles are smaller in magnitude, the distance between the joints is greater and the distance between the propshaft supports is greater. All these factors minimize the dynamic forces originating from the universal joint secondary couples and exerted on the propshaft supports.
In addition to the above noted distinctions between single and multi-piece propshafts, other factors also influence the selection of a propshaft. Single piece propshafts are simpler to engineer, and thus are less expensive to manufacture compared to multi-piece propshafts. In addition, multi-piece propshafts typically are more than double the weight of comparable length single piece propshafts.
Accordingly, what remains needed in the art is a solution to all of the above stated problems via a re-engineered propshaft assembly capable of operating at current vehicle operating speeds, and featuring: elimination of launch shudder, greatly reduced the shaft's dynamic imbalance forces, greatly improved durability life of the propshaft assembly, and reduced ability of the propshaft to transmit or amplify other vehicle generated noises such as drive axle hypoid gear noise and reducing spline friction related “clunk” or “grunt” noises.